The recent resurgence of pertussis in the United States requires a reexamination of our approach to controlling this illness. Pertussis can result in hospitalization and death, especially among infants who are too young to benefit from immunization. To reduce the burden of pertussis in infants, the Advisory Committee on Immunization Practices (ACIP) recommended in June 2011 that all pregnant women should receive the tetanus-diptheria-acellular pertussis (Tdap) booster in late pregnancy. The ACIP recommendation was informed by immunological studies showing that maternal anti-pertussis antibodies are transferred to the fetus; however, to support these immunological studies, there have been no large-scale studies showing that infant pertussis cases can be prevented by this strategy. One important concern is that passively-acquired maternal antibodies may act to blunt the infant's own immune response to the diphtheria-tetanus-acellular pertussis (DTaP) series in early infancy, and once these passively-acquired antibodies wane, the infant may be left with inferior protection. Further, little is known about the safety or uptake of the Tdap in pregnancy recommendation. This study brings together a team of investigators to evaluate the uptake, effectiveness, and safety of Tdap immunization in pregnancy. We propose a non-experimental study design using an existing commercial insurance claims database, including over 1 million women with live-birth deliveries between June 2011 and December 2013. We will first characterize the use of Tdap in pregnancy since June 2011 and examine personal, provider, and ecologic characteristics associated with adherence to the recommendation, to inform future studies on barriers to Tdap immunization. Next, we will compare the risk of pertussis hospitalization among infants less than 6 months of age who were born to a) mothers immunized with Tdap during pregnancy, b) mothers unimmunized with Tdap, and c) mothers immunized with Tdap in the postpartum period (as previously recommended). We will further examine the relationship between the timing of Tdap administration in pregnancy and the risk of pertussis hospitalization. Finally, we will evaluate the safety of Tdap in pregnancy by comparing the relative risks of adverse pregnancy and infant outcomes, such as preterm delivery, stillbirth, and congenital anomalies between groups of immunized and unimmunized pregnant women and their respective infants. We will also use novel data-driven signal detection methods during the 2 week period after Tdap administration to pregnant women to detect previously unreported adverse reactions. This proposal will advance statistical methodologies for analyzing existing data on vaccine safety and effectiveness, while at the same time providing important information on the current recommendation for infant pertussis prevention. If our findings shows that Tdap immunization in pregnancy is safe and effective at preventing infant pertussis, this may help to increase immunization coverage among pregnant women, and thereby reduce the number of infant pertussis cases and deaths.